1. Field of the Invention
The present invention relates to a mobile communication system including a sector cell arrangement, and more particularly to a mobile communication system which allocates communication channels with use of a carrier wave/interference wave power ratio (hereinafter, referred to as a C/I ratio.).
2. Prior Art
Referring to FIG. 1, a sector cell arrangement in a conventional mobile communication system is illustrated in an exemplified manner. Sector antennas provided in base stations 11, 12 form sectors A.sub.1, B.sub.1, C.sub.1 and A.sub.2, B.sub.2, C.sub.2, respectively. A situation will be described wherein a mobile station 14 moving in the sector C.sub.2 sends a call to the base station 12 which hereby allocates the same channel as a communication channel allocated by the base station 11 communicating with a mobile station 13 moving in the sector C.sub.1 through the allocated channel.
Since the same channel is used, the mobile station 14 receives not only a down-going carrier wave from the base station 12 but also a down-going interference wave 15 from the base station 11, while the base station 12 receives not only an up-going carrier wave from the base station 14 but also an up-going interference wave 16 from the mobile station 13 under communication with the base station 11. It is herein assumed that "up-going" denotes a direction from any mobile station to any base station while "down-going" denotes a direction from any base station to any mobile station.
In mobile communication systems, the following two systems have generally been proposed for allocation of dynamic channels.
In the first system, up-going interference wave power and up-going carrier wave power are measured for each sector channel to calculate an up-going C/I ratio. Further, there is selected an optimum one of channels having the up-going C/I ratios exceeding a predetermined threshold (or a quality reference), taking the channel conditions of surrounding base stations into consideration, and the selected channel is in turn allocated to an originating call.
In the second system, an up-going C/I ratio is similarly estimated and a mobile station is informed of a communication channel where the up-going C/I ratio exceeds a quality reference as a candidate channel. The mobile station measures a down-going interference wave and a down-going carrier wave on the informed candidate channel, and the measured result is sent back to the base station which estimates a down-going C/I ratio and allocates to the originating call a channel where the estimated down-going C/I ratio exceeds a predetermined threshold. In this system, once a C/I ratio of a certain channel in a cell is confirmed to exceed a predetermined quality reference, the quality of that channel is guaranteed to some degree without referring to surrounding base stations, so that there are reduced process procedures and the amount of information transfer for control of channel allocation. Such a system is disclosed in Japanese Patent Laid-Open No. 3-167924.
However, when the sectors formed by the base stations 11 and 12 are directed in the same direction, as illustrated in FIG. 1, the direction of transmission of the interference wave 16 from the mobile station 13 is out of the range of directivity of the sector antenna of the sector C.sub.2, so that a received electric field is severely reduced. Contrarily, the down-going interference wave 15 from the base station 11 is received with an omnidirectional antenna of the mobile station 14 because of the mobile station 14 being existent in the range of the directivity of the sector C.sub.1. In such a situation, in the dynamic channel allocation method of the first system wherein an optimum channel with the up-going C/I ratio exceeding the quality reference is selected and allocated to an originating call, a communication channel is allocated without taking the down-going C/I ratio into consideration, so that there is a case where any channel not satisfying the down-going quality reference is allocated to result in the loss of the call. More specifically, a receiving electric field level of the up-going interference wave 16 received by a sector antenna having directivity is smaller, so that the up-going C/I ratio is greater than an actual one.
Also in the method of the second system where a channel having both the up-going and down-going C/I ratios satisfying the quality reference is allocated, the up-going C/I ratio becomes a greater value than an actual one, as described above, so that the number of candidate channels not actually satisfying the quality reference is increased. Accordingly, a candidate channel informed from the base station 12 to the mobile station 14 sometimes fails to satisfy the quality reference of the down-going C/I ratio, resulting in an increased number of times of communication between the base station 12 and the mobile station 14. In other words, when the informed candidate channel does not satisfy the quality reference of the down-going C/I ratio, candidate channels are sequentially selected in the order previously determined at all base stations, and communication is repeated until a channel satisfying the quality reference is found.